Katie Rough was found with severe injuries on a playing field in York in January and died in hospital.
Photograph: North Yorkshire police/PA

An “extremely troubled and damaged teenager” obsessed with death has admitted suffocating and stabbing to death a seven-year-old girl in York.

The 16-year-old girl, who cannot be named, appeared by video link at Leeds crown court, where she pleaded guilty to manslaughter by diminished responsibility.

She admitted planning to kill Katie Rough, who was found with severe lacerations to her neck and chest on a playing field in January and died later in hospital. Psychiatrists have found the girl had diminished responsibility when she attacked Katie.

A friend told investigators the teenager self-harmed and liked to talk about death, and that she had said she dreamed of killing someone and heard voices in her head.

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Drawings found in the teenager’s home showed stickmen in various poses depicting killing and death and the words: “They are not human.”

The paper was bloodstained and the court heard it had been cut with the same knife that was used to attack Katie.

The teenager’s bedroom contained books, notes and comics of a violent nature. Police found a soft toy of Simba from the Lion King that had been mutilated.

The prosecution accepted that the teenager had a mental disorder at the time of Katie’s killing, which affected her ability to think and reason clearly and to control her emotions and actions.

The teenager appeared on the video link sitting next to a solicitor and wearing a black hoodie. Neither the judge nor the barristers wore their wigs or gowns because of her age.

She did not speak her plea, as is usual, but wrote it on a piece of paper, which was read to the court by her solicitor.

Graham Reeds QC, prosecuting, said: “We are going to accept that plea of manslaughter by diminished responsibility.” Reeds said the defendant had been subject to four psychiatric and psychological reports.

He said there was no dispute that her mental health problems had diminished her responsibility at the time she killed Katie, even though the killing was planned.

The court heard that the teenager was the first to call police after she attacked Katie. Shortly before, a local resident had seen the two girls on a sports and recreation area on the afternoon of 9 January.

The resident had seen both girls lying on the grass, with the teenager on top of Katie, but said both got up and Katie appeared unharmed and not in distress.

A short time later, another resident saw the teenager “distressed, covered in mud and [she] had a bloodstained right hand”, and he took her into his home.

The girl told the resident Katie was dead and asked him where she was, Reeds said. The man then found Katie lying on a nearby piece of land with a cut to her neck. She appeared to have no pulse or signs of breathing, Reeds told the court.

The teenager was arrested and appeared upset and in shock, producing a bloodstained Stanley knife she had taken from her grandmother’s kitchen. She made no comment when later interviewed by police.

A postmortem showed Katie had two severe cuts to her body – one to her neck and the other to her torso – but neither caused her death. A pathologist and a forensic scientist later established the teenager had smothered Katie, the court heard.

The teenager’s defence barrister, Nicholas Johnson QC, described her as “extremely troubled and damaged”.

He said that although she was not the victim, the 16-year-old had shown the classic symptoms of post-traumatic stress disorder.

He told the court his client had been having “delusional and bizarre thoughts” for months and had been self-harming since Christmas 2015. He said the teenager had thoughts that people around her “may not be human and may be controlled by a higher and hostile force”.

The barrister said his client had posted a picture on social media two days before the killing with a concerning message. He said: “She was clearly crying out for help and support.”

Reeds told the court the teenager had displayed “strange behaviour towards other people and herself”, and had started to self-harm before she killed Katie. She was given medication for anxiety and depression, and had been suffering from delusions in 2016, he told the judge.

The prosecutor said the girl had talked of being convinced that people “weren’t human and were robots”.

The prosecutor said the experts disagreed on her exact diagnosis but one thought she had an emerging schizotypal personality disorder but was not schizophrenic.

Reeds said: “Over a course of a year, she developed an interest in the macabre. She lost most of her friendship group at school, started to harm herself with a blade. She was frequently very upset and reported suicidal thoughts.”

Katie’s family were in court to hear the guilty plea.

The judge, Mr Justice Soole, said he wanted more questions answering by the medical experts before he could pass sentence. He adjourned the case.